It is important to measure and control the feature size of various patterns during the fabrication of semiconductors. Such a measurement is commonly known in the art as a critical dimension (CD) measurement. A critical dimension scanning electron microscope (CDSEM) is typically utilized for CD measurement of semiconductor features.
A CDSEM operates by focussing of an electron beam onto a semiconductor wafer or substrate. Backscattered and secondary electrons are generated by the electron beam and collected for measurement purposes. Scanning the electron beam across a feature of interest and detecting the backscattered and secondary electrons allows CD measurement of the feature.
Production line operators are typically instructed how to operate the CDSEM so that they may accurately perform CD measurements during various stages of semiconductor fabrication. Because the accuracy of a CD measurement depends upon how accurately the electron beam of the CDSEM is focussed onto the feature of interest on the semiconductor wafer, an emphasis is placed during instruction on how to properly focus the electron beam.
The procedure for focussing the electron beam of the CDSEM generally involves three basic steps: objective aperture alignment, focus adjustment, and astigmatism adjustment. Objective aperture alignment is especially critical because if the objective aperture is not aligned, the image of the feature will move across the display screen during focussing. The objective aperture is typically aligned by focussing the electron beam on a circular feature on a fiducial mark. If the central axis of the aperture is not aligned with the condenser lens of the CDSEM, the image of the feature will move across the display screen during focussing, as stated earlier. The central axis of the aperture may be aligned with the condenser lens using manually adjustable controls that move the aperture in the X and Y directions. The manual controls are adjusted so that the feature stays in about the same area of the display screen during focusing. When this is accomplished, the aperture is considered to be properly aligned.
Once the aperture alignment has been correctly adjusted, the CDSEM may be operated for many months by merely adjusting electron beam focus and astigmatism. Over time, however, the aperture tends to degrade thereby making proper focus and astigmatism adjustment difficult for the operator. When this occurs, the aperture must be changed and then properly aligned. Since the task of changing and aligning the new aperture is typically not released to the operator of the CDSEM, an equipment engineer must be called to change the aperture and align it manually. The down time resulting from this reduces critical dimension measurement throughput and effectively increases the measuring time per wafer.